1. Field of the Invention
The present invention relates to methods for producing an ultra-high-strength hot-rolled steel having a tensile strength of, e.g., not lower than 800 MPa, and in particular preferably not lower than 900 MPa, and being a particular degree of toughness of a base steel and at a weld heat-affected zone in the temperature range from −60° C. to 0° C. (hereunder referred to as “low temperature toughness” and “weld heat-affected zone toughness”). The present invention also relates to a method for producing a steel plate, and to a steel pipe composed of a hot-rolled steel. Such ultra-high-strength hot-rolled steels can be, after being further processed and welded, widely used for line pipes for the transport of natural gas or crude oil, pressure vessels, welded structures and the like.
2. Description of the Related Art
In recent years, a steel plate for a line pipe, for water pumping (for a penstock, for example), or for a pressure vessel is required to have improved high strength and low temperature toughness. For example, in the case of a steel plate for a line pipe, various studies have already been undertaken with regard to the production of an ultra-high-strength steel plate having a tensile strength of not lower than 800 MPa (not lower than X100 in the API standard) and high-strength steels excellent in low temperature toughness, weld heat-affected zone toughness and weldability as described in Japanese Patent Nos. 3244986 and 3262972, the entire disclosures of which are incorporated herein by reference. In addition, an ultra-high-strength line pipe having a tensile strength of not lower than 900 MPa and the Production method thereof are described in Japanese Patent publication No. 2000-199036, the entire disclosure of which is incorporated herein by reference.
However, in a steel plate for a line pipe described in the above-mentioned Japanese Patent Nos. 3244986 and 3262972, though the Charpy absorbed energy at −20° C. at a heat-affected zone to which single layer welding is applied is not lower than 100 J and thus very good, weld heat-affected zone toughness may, at times, deteriorate at a beat-affected zone to which double or more layer welding is applied under some welding conditions.
Further, in a steel plate for a line pipe described in the above-mentioned Japanese Patent Nos. 3244986 and 3262972 and also in an ultra-high-strength line pipe described in the above-mentioned Japanese Patent Publication No. 2000-199036, though the Charpy absorbed energy of a base steel at −40° is not lower than 200 J on the average when the number of the specimens (hereunder referred to as “n”) subjected to the test by using the same material and under the same test conditions is three and the result is very good, a problem may arise that the Charpy absorbed energy of some specimens is lower than 200 J, and is dispersed widely in some cases.
As a result of studying the problem of the dispersion of low temperature toughness in detail, it was ascertained that Charpy absorbed energy was lower than about 200 J with a probability of about 20 percent when Charpy impact test was performed at −40° C. under an increased number n, and further that Charpy absorbed energy of some specimens was not higher than 100 J and brittle fractured faces were observed on the fractured surfaces of the specimens when the Charpy impact test was performed in the temperature range from −60° C. to not higher than −40° C.
A method for improving low temperature toughness by contriving a welding method is described in Japanese Patent Application No. 2001-336670, the entire disclosure of which is incorporated herein by reference. However, in this publication, it was not explicitly elaborated on that that the method described therein was not immediately applicable because it was not well suitable for mass production, and preferred the introduction of new equipment. In view of the above, the development of a high-strength line pipe excellent in low temperature toughness at both a base steel and a weld is desirable.